Accessible via the UI: click into the extension, click the right arrow next to a container, select "Manage This Container", then "Advanced Proxy Settings" and you can enter a SOCKS5 URI. And of course, this requires the aforementioned extension, not just the built-in container tabs feature. (Confusingly, Firefox does have this feature "cooked in", presumably they just expose a bunch of it via WebExtension APIs so that things like Multi-Account Containers can work.)